The present disclosure relates generally to activated carbon materials and more particularly to electro-chemical double layer capacitors comprising passivated activated carbon-based electrodes.
Energy storage devices such as ultracapacitors may be used in a variety of applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Ultracapacitors typically comprise a porous separator and an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layers that are created at the interfaces between the electrodes and the electrolyte. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon that is incorporated into the electrodes.
Carbon-based electrodes suitable for incorporation into high energy density devices are known. The carbon materials, which form the basis of the electrodes, can be made from natural or synthetic precursor materials. Natural precursor materials include coals, nut shells, and biomass. Synthetic precursor materials typically include phenolic resins. With both natural and synthetic precursors, carbon materials can be formed by carbonizing the precursor and then activating the resulting carbon. The activation can comprise physical (e.g., steam) or chemical activation.
A property of the carbon that can influence its performance when incorporated into high energy density devices such as electro-chemical double layer capacitors (EDLCs) is the specific capacitance. Higher specific capacitance generally results in a higher volumetric energy density of the resulting device. In relation to the capacitance, a beneficial attribute is the ability to maintain (or not significantly lose) capacitance over time and/or as a result of multiple charge-discharge cycles that accumulate with use. Aging of the carbon materials, such as by radical or ion trapping, can reduce the useful life of ultracapacitors comprising activated carbon-based electrodes.
In addition to increasing the capacitance, because energy density is also proportional to the operating voltage of the device, higher energy densities can be achieved by operating the EDLC at higher voltages. The applied voltage can be limited, however, by physical and chemical interactions that occur at the surfaces of the carbon material. These interactions, which include Faradaic reactions that are deleterious to the functionality of the carbon, are exacerbated at higher voltages. Current state of the art EDLCs operate at about 2.7 Volts.
Accordingly, it would be an advantage to provide activated carbon materials having a high specific capacitance that are resistant to aging and which can be incorporated into EDLCs for operation at higher voltages, such as about 3 Volts. Such materials can be used to form carbon-based electrodes that enable efficient, long-life and high energy density devices.